This invention relates to the use of certain keto and diketo acids, esters and salts that are derived from aldohexoses and aldopentoses, to maintain very high levels of iron in solution under oil well acidizing conditions. The compounds described herein chelate ferric iron at low temperatures and are activated by heat to reduce ferric iron to the more soluble ferrous species at high temperatures. Because of these properties and their extraordinarily high solubility in hydrochloric acid, such compounds are capable of maintaining considerably more iron in solution than competing materials.
When the formation around oil producing wells become plugged with acid soluble minerals, the flow of fluid through the oil bearing formation is reduced and oil production falls. Wells can be stimulated to produce at higher levels by forcing acid into the formation to dissolve the minerals that are causing the problem. The acid readily dissolves iron and iron containing compounds both from the well casing and the formation. As the acid is neutralized by water and carbonates in the formation, iron will re-precipitate as ferric hydroxide above a pH of 2.2 unless suitable chemicals are added to maintain it in a soluble state.
Two approaches have been used to control reprecipitation of iron as acid is spent and the pH rises, sequestration by organic chelants and reduction to the more soluble ferrous ion. Commonly used organic chelants include citric acid, gluconic acid, the tetrasodium salt of ethylenediaminetetraacetic acid (EDTA), and the trisodium salt of nitrilotriacetic acid (NTA). Smith, et al., Secondary Deposition of Iron Compounds Following Acidizing Treatments, paper SPE (Society of Petroleum Engineers) 2358 presented at the SPE-AIME (American Institute of Mining, Metallurgical and Petroleum Engineers) Eastern Regional Meeting, Charleston, W. VA., Nov. 7-8, 1.68), have discussed the relative merits of each. Gluconic acid is ineffective at elevated temperatures. Citric acid, is effective only at low temperatures. Also, the formation of insoluble calcium citrate limits the level at which it can be used. EDTA is effective at high temperatures and is not prone to precipitation as the calcium salt. However, it is of limited solubility in hydrochloric and other acids. NTA is effective without having the negative attributes of either EDTA or citric acid. NTA, however, has been proven to cause cancers in laboratory animals. (See Chemical Status Report issued by the National Toxicology Program, Division of Toxicology Research and Testing, Oct. 8, 1991, p 25.)
While ferric iron will precipitate as the pH rises above 2.2, ferrous iron remains soluble up to a pH of about 7. Erythorbic acid and sodium erythorbate have been used commercially to control the concentration of dissolved iron in acidizing operations by reducing the ferric iron to the more soluble ferrous species. Crowe et al., in U.S. Pat. No. 4,574,050, argue that this approach is effective in maintaining substantially more iron in solution than can be maintained using the above-mentioned chelating agents.
An ideal additive for controlling the concentration of dissolved iron under oil well acidizing conditions should have a high solubility in the acid employed in the acidizing process, prevent precipitation of high levels of dissolved iron over a wide temperature range, and remain functional for several hours until the spent acid is pumped out of the formation.